With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. In the initial WLAN technology, a frequency of 2.4 GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared communication, etc. Recently, the WLAN technology can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplex (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to 540 Mbps or higher, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate.
The MIMO technique combines data streams which arrive with various time differences through various paths to effectively improve signal capability of the receiver, and thus activates a function of a smart antenna. A single input single output (SISO) technique allows one system to transmit and receive only one spatial stream at one time, whereas a MIMO technique allows transmission of multiple spatial streams. The MIMO technique can increase channel capacity in proportion to the number of antennas without additional frequency allocation or transmit power allocation. Channel capacity in a limited frequency resource is increased by using multiple antennas at both ends, and a high data rate is guaranteed.
The IEEE 802.11n standard can perform data transmission by using a channel having four spatial streams and a 40 MHz bandwidth. In this case, an equal modulation (EQM) scheme in which all streams have the same MCS level or an unequal modulation (UEQM) scheme in which each stream has a different MCS level can be used.
Recently, a channel having a bandwidth of 80 MHz is used to provide a throughput of 1 Gbps or higher, and researches for supporting multi-user (MU) MIMO are actively ongoing to enable data transmission/reception with respect to an access point (AP) by utilizing the channel simultaneously by several stations to effectively use the channel. With the use of the wider bandwidth and the support of the MU-MIMO, it is expected to use more spatial streams to perform data transmission by a station and an AP having more transmission (TX)/reception (RX) interfaces.